1. Field of the invention
The present invention relates to a radio communication apparatus and radio communication method concerning downlink CoMP transmission (Coordinated Multiple Point transmission).
2. Description of the Related Art
An LTE (Long Term Evolution) system (3GPP, TR25.912 (V7.1.0), “Feasibility study for Evolved UTRA and UTRAN”, Sept. 2006) defined in 3GPP (3rd Generation Partnership Project) adopts OFDMA (Orthogonal Frequency Division Multiple Access) for downlinks. Thus, through scheduling by a radio base station apparatus, certain frequency and time radio resources are basically allocated to one mobile terminal apparatus (UE: User Equipment), and therefore users in the same cell are orthogonal to each other in the frequency and time domains. However, since the LTE cellular system is based on frequency reuse one, large interference is received from surrounding cells and the interference level at a UE located in a cell edge is particularly high. Measures against inter-cell interference are required to compensate for such peripheral cell interference and maintain constant receiving quality.
3GPP is studying an LTE-A (LTE-Advanced) system to realize high-speed transmission with broader coverage than the LTE cellular system. The LTE-A (LTE-Advanced) system adopts a coordinated multiple point transmission/reception (CoMP) technique as an additional inter-cell interference control (ICIC). There are two methods for realizing the CoMP technique; autonomous distributed control based on an independent eNB (eNode B) configuration (inter-eNB coordination) and concentrated control based on a remote base station (RRE: Remote Radio Equipment) configuration (intra-eNB coordination). The independent eNB configuration requires signaling through a wired transmission path (X2 interface) between eNBs as shown in FIG. 1.
In an inter-base-station coordinated transmission system such as CoMP, data and reference signals (RS) are simultaneously transmitted, for example, on downlinks from a plurality of (here, two) radio base station apparatuses eNB#1 and eNB#2 to mobile terminal apparatuses UE#1 and UE#2 as shown in FIG. 1. Furthermore, channel information (CSI: Channel State Information) is fed back from the mobile terminal apparatus UE to the radio base station apparatus eNB.
Downlink CoMP transmission can be roughly divided into two categories; CS/CB (Coordinated Scheduling/Coordinated Beamforming) shown in FIG. 2 and JP (Joint Processing) shown in FIG. 3. With CS/CB as shown in FIG. 2, data is transmitted to a specific UE (UE#1) from one cell (eNB#2) in a subframe, and beam forming and scheduling coordinated among a plurality of cells are performed so as to reduce interference with the other UE (eNB#1) in the other cell. On the other hand, with JP as shown in FIG. 3, transmission is performed simultaneously for a specific UE (UE#1 or UE#2) from a plurality of cells (eNB#1, eNB#2) using the same time and frequency radio resources (joint transmission). In this case, precoding is performed between cells and signals are in-phase combined at a receiving point (coherent combining).
With CS/CB or JP, a CSI measured value for each user is fed back to the radio base station apparatus. In this case, making the most of the effect of CoMP (avoidance of spatial interference between cells) requires spatial information of the channel to be fed back accurately from the mobile terminal apparatus. This causes the amount of feedback information from the mobile terminal apparatus to the radio base station apparatus to increase. Furthermore, in the case of coordinated transmission between a plurality of base stations as shown in FIG. 1, feedback information or data needs to be shared between the radio base station apparatuses via the X2 interface, which results in a problem that a large capacity X2 interface will be required when causing many radio base station apparatuses to coordinate with each other.